To summary or not to summary: EIGRP

One of the key features of EIGRP is that it auto-summarizes routes by default. It is interesting to spot the differences in the routing table among equipments when this feature is activated or deactivated.

I will propose for this post the following topology, used by my teacher in a CCNP-ROUTE lab as a case study:

the config of the devices goes as follows:

for R1:

interface Loopback0
 ip address 10.2.11.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 10.10.10.1 255.255.255.252
!
router eigrp 65500
 network 10.2.11.0 0.0.0.255
 network 10.10.10.0 0.0.0.3

For R2

interface Loopback0
 ip address 172.16.1.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 10.10.10.2 255.255.255.252
!
interface FastEthernet0/1
 ip address 20.20.20.2 255.255.255.248
!
router eigrp 65500
 network 10.10.10.0 0.0.0.3
 network 20.20.20.0 0.0.0.7
 network 172.16.1.0 0.0.0.255

For R3:

interface Loopback0
 ip address 30.30.30.1 255.255.255.240
!
interface FastEthernet0/0
 ip address 20.20.20.1 255.255.255.248
 duplex auto
 speed auto
!
!
router eigrp 65500
 network 20.20.20.0 0.0.0.7
 network 20.30.30.0 0.0.0.15

 when EIGRP is activated in its default mode (auto-summary), the routing tables are seen like this:

Looking at R1's printout, you may see R2's and R3's loopbacks with its default mask (8 bits and 16bits respectively. Why is that? because R2 and R3 are summarizing the networks they receive from EIGRP.
R2 will summarize 20.20.20.0/29 to R1 as 20.0.0.0/8, because the out interface from R2 to R1 has an ip address that is not from the same family (10.10.10.0/30).
this behaviour applies also to R2's loopback. R1 sees this route as 172.16.0.0/16, because this IP does not belong to the same family as 10.10.10.0/30.

In R2, you see R3's loopback as 20.30.30.0/28. Why this route appear with the original mask? because R3 does not summarize this route. The IP address of the interface that goes from R3 to R2 (20.20.20.0/29) belongs to the same family of the loopback (20.30.30.0/28)

Conclussion: summarization for EIGRP will only take place if the network to be announced does not belong to the same family of the out interface.

There is a tip to know when a router is summarizing: the router shows NULL0 as the out interface. This is useful as a protection mechanism: the router will search for an exact match in its routing table when finding a destination to a packet. If this exact match does not exist, it will discard the packet, to the Null interface.

Lets apply the no auto-summary command in each EIGRP process. At the end of the process, the output of the show ip route command goes as follows:

You will no see routes to Null0 any more. You will see the routes with its original network mask. Summarization is not performed.